The present invention relates to a conductive ceramic element and, more particularly, to a ceramic element with terminal electrodes having a baked electrode layer, and an insulating inorganic layer encapsulating the surface of the ceramic body between the terminal electrodes.
A conductive chip-type ceramic element according to the prior art includes a ceramic body having silver-palladium electrodes fused at both ends thereof. The palladium imparts soldering heat resistance to the electrode, thereby preventing the silver from dissolving when soldering a chip-type thermistor to a substrate.
A drawback of the prior art is that the terminal electrode is manufactured by dipping the ends of the ceramic body into a silver-palladium paste. Regardless of the level of control exercised during the dipping process, however, dispersion of the material of the electrode near the edges of the thermistor body is inevitable. This produces a dispersion in the part-to-part resistance values of the resulting elements.
A further drawback of the prior art is that the increased palladium content causes a decrease in the solder adhesion of the electrode to the substrate, thereby establishing an upper limit on the amount of palladium which can be used. When soldering an electrode at high temperature for a long period of time, however, this upper limit provides insufficient palladium to impart adequate soldering heat resistance to the electrode.
It has been suggested that soldering heat resistance and soldering adhesion can be improved by providing a plating layer on the surface of the electrodes, as in the case of a chip-type capacitor. A drawback of this suggestion is that since the ceramic body of a thermistor is conductive (unlike the ceramic body of a capacitor), plating a conductive material directly on the surface of the ceramic body alters the resistance value of the ceramic element from the desired value. In addition, a portion of the ceramic is eroded by the plating liquid, thereby reducing the life and reliability of the ceramic element.
To overcome the above drawbacks, Applicant has filed Japanese Laid-Open Patent Publication No. 3-250,603, laid open in 1991, for a chip-type thermistor shown in FIG. 19. A ceramic body 10 is covered by a glass layer 4 and a baked electrode 16. A plated layer 3 covers baked electrode 16. Plated layer 3 is prevented from direct contact with ceramic body 10 by glass layer 4.
A drawback to Applicant's previous invention is that the sintered ceramic body is formed by coating both sides of a sheet of ceramic with glass. The glass-coated ceramic is then cut into pieces. This process requires numerous process steps and careful handling, thereby resulting in high manufacturing costs.
When forming a terminal electrode having plating layers on a conductive chip-type ceramic body, another conventional method utilizes ceramic material with a low conductivity. A drawback of this is that a low-conductivity chip requires a larger body due to its low magnetic permeability, which interferes with high density mounting. A substitution of a ferrit based ceramic material with a higher magnetic permeability avoids this inconvenience, but in turn experiences a shift in other characteristics due to plating material adhering to the ceramic body.
To overcome the above drawbacks, Applicant has filed Japanese Laid-Open Patent Publication No. 3-250,601 for a chip-type thermistor. Referring now to FIG. 20, an inner electrode 111 is formed on the end surfaces of ceramic body 10. A copper outer electrode 116 covers inner electrode 111.
A drawback to this approach is that a copper outer electrode 116 imparts significantly less heat resistance than a plated nickel electrode. The need for an outer layer that contacts portions of the surface of the thermistor body to provide good heat resistance, has limited the material for the outer electrode. This in effect complicates the conditions for burning.